1. Field of the Invention
The present invention relates to a paper processing belt used to carry out the transfer of a paper sheet between sections, or between elements of a section, such as the individual presses in a press section, of the paper machine on which it is being manufactured, or to carry the sheet into other processes. Specifically, the present invention is a transfer belt which may be joined into endless form during its installation on a paper machine with a seam, and a method for closing the seam region after the coated seamed transfer belt has been so joined.
2. Description of the Prior Art
At present, the only commercially available paper processing belt of this type is a transfer belt. A transfer belt is designed both to carry a paper sheet through a portion of a paper machine, so as to eliminate open draws from the machine and to release the sheet readily to another fabric or belt at some desired point. By definition, an open draw is one in which a paper sheet passes without support from one component of a paper machine to another over a distance which is greater than the length of the cellulose fibers in the sheet and is susceptible to breakage. The elimination of open draws removes a major cause of unscheduled machine shut-down, the breakage of the sheet at such a point where it is temporarily unsupported by a felt or other sheet carrier. When disturbances in the flow of paper stock occur, the likelihood of such breakage is quite strong where the unsupported sheet is being transferred from one point to another within the press section, or from the final press in the press section to the dryer section. At such points, the sheet usually is at least 50% water, and, as a consequence is weak and readily broken. Clearly, the presence of an open draw will place a limitation on the maximum speed at which the paper machine may be run.
A successful sheet transfer belt must carry out three critical functions on the paper machine: a) to remove the paper sheet from a press fabric without causing sheet instability problems; b) to cooperate with a press fabric in one or more press nips to ensure optimal dewatering and high quality of the paper sheet; and c) to transfer the paper sheet in a closed draw from one press in the press section to a sheet-receiving fabric or belt in the next press, or presses, in the press section, or to a dryer pick-up fabric in the dryer section.
A sheet transfer belt which successfully carries out these critical functions is shown in U.S. Pat. No. 5,298,124, entitled "Transfer Belt" and issued on Mar. 29, 1994, the teachings of which are incorporated herein by reference. The transfer belt shown therein has a surface topography characterized by a pressure-responsive, recoverable degree of roughness, so that, when under compression in a press nip, the degree of roughness will decrease, thereby enabling a thin continuous water film to be formed between the transfer belt and a paper sheet to bond the paper sheet to the transfer belt upon exit from the press nip. When the original degree of roughness is recovered after exit from the nip, the paper sheet may be released by the transfer belt, perhaps with the assistance of a minimum amount of vacuum, to a permeable fabric, such as a dryer pick-up fabric.
The sheet transfer belt shown in that application comprises a reinforcing base with a paper side and a back side, and has a polymer coating, which includes a balanced distribution having segments of at least one polymer, on the paper side. The balanced distribution takes the form of a polymeric matrix which may include both hydrophobic and hydrophilic polymer segments. The polymer coating may also include a particulate filler. The reinforcing base is designed to inhibit longitudinal and transverse deformation of the transfer belt, and may be a woven fabric, and further may be endless or seamable for closing into endless form during installation on the paper machine. In addition, the reinforcing base may contain textile material, and may have one or more fiber batt layers attached by needling onto its back side. By textile material is meant fibers and filaments of natural or synthetic origin, intended for the manufacturing of textiles. The back side may also be impregnated and/or coated with polymeric material.
To date, such sheet transfer belts have been produced for paper mills in endless form, that is, having reinforcing bases either woven in endless form or joined into endless form prior to being coated with the polymer material. The installation of an endless transfer belt on a paper machine, however, is a time-consuming and technically complicated endeavor. It goes without saying that paper production must temporarily cease while the transfer belt installation, or replacement, proceeds. Because the installation of an endless belt cannot be accomplished by snaking or threading the belt through and around the components of the paper machine, it must be inserted from the side of the machine. This necessarily is much more time- and labor-intensive than the installation of an open-ended belt, as machine components, such as press rolls, must be supported while the transfer belt is slipped into the spaces between them from the side. Needless to say, the provision of a sheet transfer belt which may be seamed on the machine would significantly reduce the time and labor required to install, or replace, one on a paper machine.
International Publication No. WO 93/17161, disclosing International Application No. PCT/SE93/00173, shows a joinable band comprising a textile web which is provided from at least one side and through at least part of its thickness with a quantity of thermoplastic material. When heat-softened, the thermoplastic material will fill out the fabric structure of the web at least partially. The edges of respective ends of the band have joining eyelets, which are formed in the textile web and which coact with joining eyelets similarly formed in a meeting end of the band so as to form a detachable join. In order to enable the band to be fitted easily to a machine and to provide the region of the band join with the same properties as the remainder of the band, no plastic filler is applied to the textile web joining means along a region whose width extension calculated from end edge and inwardly of the web corresponds at least to the extension of the eyelets over that part which coacts with the eyelets of the meeting web end.
The difficulty associated with the provision of an open-ended, or seamable, transfer belt is the marking likely to be left on the paper sheet by the seam region. Because the sheet transfer belt carries a paper sheet through a press nip, and is in direct contact with the paper sheet therein, the slightest difference in caliper, compressibility and surface hardness of the seam region of the belt will leave a mark on the sheet.
Accordingly, the principal object of the present invention is to provide a seamable sheet transfer belt, and a method for making the same, wherein the seam region thereof has properties substantially identical to those of the remainder of the sheet transfer belt, so that the seam region may not mark the paper sheet.
It is also an object of the present invention to provide a seamable sheet transfer belt, so that the time and labor required to install or replace such a belt on a paper machine may be reduced.
It is a further object of the present invention to provide a seamable sheet transfer belt, so that existing paper machines may be more readily modified, or adapted to incorporate, the sheet transfer belt shown in U.S. Pat. No. 5,298,124, whereby open draws may be eliminated therefrom.